Reorder point + safety stock calculator
The reorder point calculator returns ROP = d * L + Z * sigma_LT, where d is average daily demand, L is lead time in days, sigma_LT is the standard deviation of demand during lead time, and Z is the standard normal score for your chosen service level[1]. Z values: 90% -> 1.282, 95% -> 1.645, 97.5% -> 1.96, 99% -> 2.326, 99.5% -> 2.576.
Choosing service level
Service level is the probability of not stocking out during lead time. 95 percent is the default for B-class items; A-class critical items typically target 97.5 to 99 percent; C-class items can run at 90. Each step up the service ladder costs more in safety stock because Z grows non-linearly: 95% -> 97.5% is a 19 percent SS increase, and 97.5% -> 99% is another 19 percent on top[2].
ROP and Q* are independent decisions
A common confusion: Q* (from EOQ) and ROP (from this calculator) are decided separately. Q* answers "how much do I order each time?" ROP answers "when do I trigger the next order?" In a continuous review (Q, r) policy, you set both; in a periodic review (s, S) policy, the variables collapse differently. See ROP concept page for the policy distinction.
If lead time is itself variable
When both demand and lead time vary, the combined variance is sigma_DLT^2 = L * sigma_D^2 + d_bar^2 * sigma_LT^2. Use sigma_DLT in place of sigma_LT in the safety-stock formula. The lead-time variability page walks the derivation in full.